Unmasking the mechanisms for Vernier acuity: evidence for a template modelfor Vernier acuity

The goal of this study was to evaluate the mechanisms underlying Vernier ac uity, over a range of spatial scales using narrow-band Vernier stimuli and oblique masking. Specifically, the test stimuli consisted of a pair of vert ical ribbons of horizontal cosine grating with a vertical Vernier offset be tween the ribbons. These stimuli have two important advantages for studying Vernier acuity: (1) they are relatively well localized in vertical spatial frequency, and (2) they are localized in their horizontal extent (width). We measured the orientation, spatial frequency and width tuning of Vernier acuity over a wide range of ribbon spatial frequencies, using a simultaneou s oblique masking paradigm. Our masking results suggest that the mechanisms underlying Vernier acuity are tuned to the orientation, spatial frequency and width of the ribbon stimuli. The peak of the bimodal orientation tuning function varies systematically with the spatial frequency of the ribbon. T he peak of the spatial frequency tuning function varies systematically with both the ribbon spatial frequency, and the ribbon width (i.e. the grating length). A 'template' model, in which the 'mechanism' is a windowed version of the stimulus is able to account for many features of the data, includin g results which cannot be easily accounted for by standard multi-scale filt er models. Specifically, the template model can account for: (i) the bimoda l orientation tuning function, (ii) the systematic variation in the peak of the orientation and spatial frequency tuning functions with spatial freque ncy, and (iii) the systematic effect of ribbon width on spatial frequency t uning. (C) 2000 Elsevier Science Ltd. All rights reserved.